MORPHOSTRUCTURAL ANALYSYS OF OAXACA, MEXICO. APLIED TO SEISMIC STUDIES

Morphostructural analysis, based on the determination of tectonic structural elements through the study of land forms and characteristics of the relief, was used to investigate the relation between seismicity and the tectonic structure of the Oaxaca region in SW Mexico. Topographic maps, geologic maps on a 1:1,000,000 scale, and other available geologic data were used to identify the tectonic structures of this region. The main structural elements include domes, tectonic blocks and lineaments of different magnitudes. The development of and relations between these structural elements were also inferred. We differentiate active from inactive fault zones and from fault zones that have been reactivated. We suggest a correspondence between the seismic activity and the tectonic structures found in this region by morphostructural analysis; we also suggest that the epicenters of large earth-quakes might be located, within the accuracy of determination, at the intersections of active fault zones N65° ± 5°W and important reactivated fault zones oriented N-S and E-W. The fore-shock and aftershock seismic activities associated with the 29 Novembre, 1978, earthquake (M s =7.8) show a good correlation with the details of the tectonic structure of Oaxaca. Foreshock activity is probably related to the first (after the Middle Pacific Trench) N65° ± 5°W structure on the mainland; aftershocks are presumably linked to N-S and E-W structures inland, which are related to block accomodations due to subduction movements originated by the mainshock.